1. Field of the Invention
This invention relates to a method for formation of a functional film, particularly a functional deposited film which is useful for uses such as semiconductor devices, photosensitive devices for electrophotography, electronic devices such as optical input sensor devices for optical image inputting devices, etc.
2. Related Background Art
In the prior art, for amorphous or polycrystalline functional films such as semiconductor films, insulating films, photoconductive films, magnetic films or metal films, individually suitable film forming methods have been employed from the standpoint of desired physical characteristics, uses, etc.
For example, for formation of silicon deposited films such as amorphous or polycrystalline non-single crystalline silicon which are optionally compensated for lone pair electrons with a compensating agent such as hydrogen atoms (H) or halogen atoms (X), etc., (hereinafter abbreviated as "NON-Si (H,X)", particularly "A-Si (H,X)" when indicating an amorphous silicon and "poly-Si (H,X)" when indicating a polycrystalline silicon) (the so-called microcrystalline silicon is included within the category of A-Si (H,X) as a matter of course), there have been attempted the vacuum vapor deposition method, the plasma CVD method, the thermal CVD method, the reactive sputtering method, the ion plating method, the optical CVD method, etc. Generally, the plasma CVD method has been widely used and industrialized.
However, the reaction process in formation of a silicon deposited film according to the plasma CVD method which has been generalized in the prior art is considerably complicated as compared with the CVD method of the prior art, and its reaction mechanism involves not a few ambiguous points. Also, there are a large number of parameters for formation of a deposited film (for example, substrate temperature, flow rate and flow rate ratio of the introduced gases, pressure during formation, high frequency power, electrode structure, structure of the reaction vessel, speed of evacuation, plasma generating system, etc.). By use of such a large number of parameters, the plasma may sometimes become unstable, whereby marked deleterious influences were exerted frequently on the deposited film formed. Besides, the parameters characteristic of the device must be selected for each device and therefore under the present situation it has been difficult to generalize the production conditions.
On the other hand, for the silicon deposited film to exhibit sufficiently satisfactory electric and optical characteristics for respective uses, it is now accepted the best to form it according to the plasma CVD method.
However, depending on the application use of the silicon deposited film, bulk production with reproducibility must be attempted with full satisfaction of enlargement of area, uniformity of film thickness as well as uniformity of film quality, and therefore in formation of a silicon deposited film according to the plasma CVD method, enormous installation investment is required for a bulk production device and also management items for such bulk production become complicated, with a width of management tolerance being narrow and the control of the device being severe. These are pointed as the problems to be improved in the future.
Also, in the case of the plasma CVD method, since plasma is directly generated by high frequency or microwave, etc., in the film forming space in which a substrate on which film is formed is arranged, electrons or a number of ion species generated may damage to the film in the film forming process to cause lowering in film quality or non-uniformization of film quality.
As an improvement of this point, the indirect plasma CVD method has been proposed.
The indirect plasma CVD method has been elaborated to use selectively the effective chemical species for film formation by forming plasma by microwave, etc., at an upstream position apart from the film forming space and transporting the plasma to the film forming space.
However, even by such a plasma CVD method, transport of plasma is essentially required and therefore the chemical species effective for film formation must have long life, whereby the gas species which can be employed are spontaneously limited, thus failing to give various deposited films. Also, enormous energy is required for generation of plasma, and generation of the chemical species effective for film formation and their amounts cannot be essentially placed under simple management. Thus, various problems remain to be unsolved.
As contrasted to the plasma CVD method, the optical CVD method is advantageous in that no ion species or electrons are generated which damage to the film quality during film formation. However, there are problems such that the light source does not include so many types, that the wavelength of the light source tends to be toward UV side, that a large scale light source and its power source are required in the case of industrialization, that the window for permitting the light from the light source to be introduced into the film forming space is coated with a film during film formation to result in lowering in dose during film formation, which may further lead to shut-down of the light from the light source into the film forming space.
As described above, in formation of silicon deposited film, the points to be solved still remain, and it has been earnestly desired to develop a method for forming a deposited film which is capable of bulk production by attempting to effect energy saving by means of a device of low cost, while maintaining the characteristics as well as uniformity which are practically available. These are also applicable for other functional films such as silicon nitride films, silicon carbide films, silicon oxide films as the similar problems which should be solved respectively.